This invention relates to navigation apparatus in which the position and/or route of a moving body such as an aircraft (whether it be a manned or unmanned air vehicle) is determined by comparing an external viewed scene with previously prepared references. Such a process is usually termed "scene matching".
Navigation is concerned with determining a route from a given starting point to a desired destination. One navigation system now widely used for aircraft navigation is the Inertial Navigation System (INS). This estimates the position of an aircraft by monitoring and integrating its velocity and attitude changes. Another proposed system, known as the Global Positioning System (GPS), uses radio transmissions from a network of satellites. By knowing the predetermined positions of the satellites, the position of the vehicle may be estimated.
Although GPS navigation can be very accurate, its operation relies on active radio signals, which may cause problems in certain applications. INS is passive, but cannot achieve the accuracy of GPS. Also, cost increases rapidly with increasing accuracy. As a result of these factors, various new techniques for very accurate navigation have been investigated. Scene matching is one of the most promising techniques that can be combined with an INS (or other suitable system,) to give very high accuracy autonomous navigation which is also economical.
Scene matching may be implemented in two basic configurations; either the scene can be smaller than the reference, or it can be larger than the reference. In each case, imagery of the ground beneath the aircraft (the "scene(s)") is compared with a pre-stored "reference" in order to estimate the position of the former relative to the latter. If the co-ordinates of the reference are known then the co-ordinates of the aircraft can be estimated.
This comparison, or matching, takes the form of a cross-correlation between the scene(s) and the reference. One scene at a time may be correlated and generally there is some processing of the scene before it is correlated. The cross-correlation generates a surface called the correlation surface, where the numerical value at any point on the surface is an indication of the degree of match that exists between the scene and reference for that particular positioning of the one relative to the other.
The Small Reference/Large Scene system operates well with a low density of terrain features, and benefits from being able to concentrate on a unique feature for each match. On a particular planned route, a scene is captured each time the navigation system indicates that a predetermined waypoint is being overflown. The position fix thus obtained is then fed back into the navigation computer. The main disadvantage of this method is the relatively limited tolerance of across-track drift. The importance of this depends upon the application.
One method of dealing with larger errors (both across-track and along-track) is to use the Large Reference/Small Scene System. Previously, this has involved the matching of one scene (either square or rectangular in shape) against the stored reference. The major problem with this, however, is the possibility of a false correlation, i.e. the position of maximum cross-correlation may not correspond to the geographical location of the captured scene. This results in the estimated position of the aircraft being incorrect.